An epoxy resin may be in the form of a liquid or of a solid, depending on its epoxy equivalent weight. A solid epoxy resin may be used for coatings by applying a solution of it to an object and evaporating the solvent (see Australian Pat. No. 205,972) or by melting a catalyzed powder on the object. But it is not practical to use a solid epoxy resin as a potting or casting compound since a solvent cannot evaporate from deep within the casting and a melt of the resin would tend to polymerize prematurely.
While liquid epoxy resins would therefore be much more suitable as casting and potting compounds than solids resins, until now it has been difficult to control the flexibility of liquid epoxy resins. Plasticizers have been tried but they are extractable and the resin gradually becomes less and less flexible. Flexible hardeners have also been tried but they are expensive and give only a narrow range of flexibility.
While drying oils are easily made compatible with solid epoxies (see Canadian Pat. No. 518,956 and Australian Pat. No. 205,972), efforts to incorporate drying oils into liquid epoxy resins to flexibilize them have failed to produce a clear resin.